Multichip module (MCM) substrates are used to interconnect a large number of integrated circuit chips. These boards typically have to convey a large number of signal lines between the chips. For this purpose, a typical MCM board has several layers of signal lines separated by interleaving dielectric layers, and via connections running through one or more dielectric layers perpendicular to the layer surface, as required by the specific electric interconnect network of the MCM. The signal and dielectric layers are manufactured in a "built-up" process where these thin layers are sequentially formed on a base substrate using conventional semiconductor processing techniques. Dielectric layers are typically formed by the sequential steps of spin-coating the dielectric material, curing, pattern-etching with a plasma etch process to form via apertures, and filling the apertures by electroplating or sputtering. The metal layers are typically formed by sequential steps of sputtering a thin chromium layer (for adhesion to the dielectric layer), sputtering a initial copper layer over the chromium layer, defining the electrical traces by either additive or subtractive methods, and removing the excess copper and chromium between the electrical traces. Typical additive methods use a thin initial copper layer (called a seed layer), then form a photoresist layer over the thin copper layer and pattern it to remove photoresist where the signal traces are to be located, and thereafter plate a much thicker copper layer into the photoresist pattern. Typical subtractive methods use a thick initial copper layer, then form a photoresist layer over the thick copper layer and pattern it to remove photoresist where the signal traces are not to be located. Thereafter exposed copper is etched away.
The build up process involves many steps and is expensive. A defect in the formation of one layer ruins the entire substrate. Current trends in the industry are toward increasing the density of signal lines and vias. This, in turn, increases cost of the build-up process and increases the chances of a defect occurring.
The present invention is directed to substrate structures and methods for making the same which reduce the manufacturing costs and defects, and thus enable board manufacturers to keep pace with the current trends in the industry.